Macro cargo::try_old_curl_http2_pipewait

macro_rules! try_old_curl_http2_pipewait {
    ($multiplexing:expr, $handle:expr) => { ... };
}

Enable HTTP/2 and pipewait to be used as it’ll allow true multiplexing which makes downloads much faster.

Currently Cargo requests the http2 feature of the curl crate which means it should always be built in. On OSX, however, we ship cargo still linked against the system libcurl. Building curl with ALPN support for HTTP/2 requires newer versions of OSX (the SecureTransport API) than we want to ship Cargo for. By linking Cargo against the system libcurl then older curl installations won’t use HTTP/2 but newer ones will. All that to basically say we ignore errors here on OSX, but consider this a fatal error to not activate HTTP/2 on all other platforms.

pipewait is an option which indicates that if there’s a bunch of parallel requests to the same host they all wait until the pipelining status of the host is known. This means that we won’t initiate dozens of connections but rather only one. Once the main one is opened we realized that pipelining is possible and multiplexing is possible. All in all this reduces the number of connections down to a more manageable state.